A conventional form of solar control window film consists of a substantially transparent flexible polymer substrate having a thin layer of reflective metal deposited thereon, for example, by vapor deposition or sputter deposition. The film is customarily affixed to the interior surface of a window by a substantially transparent layer of pressure sensitive adhesive. The adhesive customarily contains ultraviolet energy absorbers to protect from ultraviolet damage the contents of the room or space in which the window is located.
Depending upon the selection of the metal or metals and the thickness of the metal layer, the film will have a selected visible light transmission (VLT) and a selected visual light reflection (VLR). In general, VLT and VLR are inversely proportional. If the thickness of the metal layer is increased, VLR is increased and VLT is decreased. In order to achieve an acceptable level of solar energy rejection in most climates, the metal layer must be sufficiently thick and dense that visible light transmission is below 50%, frequently 25% or less. Thus, VLT and VLR become competing interests without a middle of the road compromise acceptable to the industry.
One attempt to increase the VLT of metalized films has been to apply coatings of titanium oxide or indium tin oxide adjacent the film or layer of metal to control reflection within a narrow spectral band. According to classic optics, sandwiching of the metal film between layers of a material of high refractive index can boost visible transmission, that is, so-called induced transmission. In common practice, this requires 70 to 100 nanometer thick layers of titanium oxide or indium tin oxide, which are very slow to produce and difficult to control. As a result, this approach is generally too expensive to be practical for many window film applications.
U.S. Pat. No. 4,799,745 (Reexamination Certificate B1 4799745) discloses an infrared reflecting film employing Fabry-Perot interference filters comprised of five or more odd numbers of alternating layers of dielectric and metal; specifically, two or more optically transparent layers of metal, such as silver, gold, platinum, palladium, aluminum, copper, nickel and alloys thereof, sandwiched between and separated by directly contiguous dielectric spacer layers, which may suitably be the oxides of indium, tin, titanium, silicon, chromium and bismuth. Related U.S. Pat. No. 5,071,206, which issued on a continuation in part of U.S. Pat. No. 4,799,745, discloses a color corrected infrared reflecting film comprised of a substrate bearing seven directly contiguous alternating layers of dielectric and silver. While these films provide the desired visible light transmission, they require 5, 7 or a greater odd number of layers of material sputter deposited onto one another, which is very costly and not easy to achieve.
The basis for the approach of U.S. Pat. No. 4,799,745, and also of U.S. Pat. Nos. 5,071,206, 4,337,990 and 3,682,528, is to maximize infrared reflection which, when the film is affixed to a window or employed in a glazing system, causes the reflected infrared energy, i.e., the solar heat, to be absorbed in the rigid support material, namely the window glass. Excessive absorption of solar heat can result in breakage of the glass. Another disadvantage of this approach is the inherently low moisture vapor transmission rate (MVTR) of the metal/dielectric stack, which typically results in excessively long drying times for the pressure sensitive adhesive attachment or installation system employed to affix the film to a window. In many cases, this can result in fogginess or haze and disrupt window aesthetics after installation.
Another approach to selective filtering of the solar spectrum has been through the use of near infrared absorbing dyes. One example is a film incorporating or coated with infrared absorbing dyes that is available from Nippon Kayaku Kabushiki Kaisha of Japan. Primary glass manufacturers also employ inorganic oxides to absorb solar heat. Examples include PPG Industries "Azurelite" glass and Libby Owens Ford "Evergreen" glass. Again, due to solar heat absorption, very high glazing temperatures are reached, promoting glass breakage, decreasing dual pane insulated glass lifetime, causing sealant failure, and producing an overall inefficient system.